1. Field of the Invention
The present invention relates to a liquid developer for developing latent electrostatic images, and more particularly to a liquid developer composed of (i) a carrier liquid comprising at least a phenylmethyl silicone or a cyclic dimethyl polysiloxane, and (ii) toner particles dispersed :in the carrier liquid.
2. Discussion of Background
A liquid developer for developing latent electrostatic images is generally prepared by dispersing a toner comprising a coloring agent and a resin in a carrier liquid which has high insulating properties and a low dielectric constant, together with a polarity controlling agent when necessary.
Conventionally, cyclohexane, decalin, kerosine, aliphatic hydrocarbon liquids, gas oil, gasoline and fluorinated hydrocarbons are available as the carrier liquids of the liquid developer.
Generally the carrier liquid for the liquid developer must have the following properties: (1) a high electrical resistivity of 10.sup.14 .OMEGA.cm or more; (2) a low dielectric constant of 3 or less; (3) a low, solubility with a solubility coefficient of about 7; (4) no odor; transparent; (6) not irritating to the skin; (7) nontoxic; (8) low flammability; and (9) quick-drying. With the aforementioned requirements for the carrier liquid taken into consideration, aliphatic hydrocarbon liquids, for example, commercially available "Isopar" (Trademark), made by Exxon. Chemical Japan Ltd., are used in practice.
Recently, there is a demand for a wet-type copying apparatus capable of yielding images with high image density even when many different kinds of transfer sheets are used. In order to obtain high image density, conventionally a large quantity of toner is caused to be deposited on a transfer sheet. However, when the amount of toner to be deposited on the transfer sheet is increased, it is necessary that the temperature of a heat-application roller in an image fixing unit be elevated to obtain better image fixing performance. Furthermore, it is necessary to employ a carrier liquid with high volatility for the liquid developer. However, when a liquid developer containing such a carrier liquid with high volatility is used for making a large volume of copies in a closed room, the vapor of the carrier liquid eventually builds up in the room and as the concentration of the vapor is increased, the vaporized carrier liquid gives out an offensive or unpleasant odor and becomes unsafe to the human body. From this point of view, utmost care must be taken to obtain good ventilation of the room when the copying apparatus is in operation. In addition, it will be necessary to control the number of copies to be made at one time or the operating speed of the copying apparatus.